1. Field of the Invention:
The invention relates to a knife holder or cutting blade for a longitudinal slitter or cutter, a rotary knife disc of which is displaceable relative to a second rotary knife disc cooperating therewith, by separate pneumatic linear drive members which are however adapted to be fed from a common controllable feed line, from an initial position, in which the knife discs are out of engagement, against a resilient restoring force, initially radially and then axially, there being a flow-limiting member connected upstream of the linear drive member for the axial feed movement. With such longitudinal slitters, material webs, for example of paper, are divided longitudinally into individual strips.
2. Description of the Prior Art:
In the known constructions of longitudinal slitters of the kind with which the invention is concerned, the problem exists of bringing the two knife discs into their slightly overlapping operative positions from above and below, respectively of the material web that is to be cut, in a disturbance-free manner and extremely carefully, but in so doing bringing them reliably into operative engagement with one another. Whilst the lower knife disc is usually cup-shaped in configuration and is ground at its front edge, the top knife disc is usually flat and sharpened (or bevelled) at its periphery. It is imperative that this top knife disc be prevented from colliding radially with the lower knife disc, which is why the feed movement is ordinarily sub-divided into a radial feed movement and an axial feed movement, the latter of which must terminate only after the first radial movement. Also the sensitive edge of the top knife disc must be prevented from colliding in the axial direction hard against the front or end edge of the lower knife disc, whilst on the other hand an adequate pressure is necessary for satisfactory cutting.
In one known version of a knife holder for a longitudinal slitter or cutter, an attempt has been made to overcome the above discussed problem in that the spring forces counteracting the two linear drive members, which are in the form of cylinders, for the radial and axial feed movement respectively, and above all the bias thereof have been coordinated with one another, taking into account the respective piston areas, in such a way that both linear drive members become active only one after the other, with desired force. This coordination is, however, suspect in that the springs age and also the frictional resistance cannot be exactly calculated, even though the cylinder for the axial feed movement has been given a complex low-friction mounting. The friction forces, in combination with the elastic pressure and spring forces have, in addition thereto, a tendency to cause a slip-stick effect which may lead to sudden and heavy collision of the knife discs.
In another known version, the pneumatic pressure medium reaches the linear drive member for axial feed movement by way of a throttle member, so that this drive member responds with a time delay. However, this time delay, too, as well as the relevant feed speed and the knife pressure (or squeezing pressure), is dependent upon the existing pressure and upon the resilient restoring force, with which the response pressure of the drive member does indeed vary. In addition, here, too, the force equilibrium between pressure force and resilient force is disturbed by friction forces in the sense of a slip-stick effect.
A further knife holder known from German Offenlegungsschrift No. 28 21 956, has a reversing valve connected prior to the linear drive member, designed as a double-acting cylinder, for the axial feed movement, which reversing valve is dependent upon the pressure in the controlled feed line of the linear drive member for the radial feed movement and by way of which either the one cylinder side is feedable directly from the controlled feed line or else, for the relevant feed movement, the other cylinder is feedable by way of a reducing valve from a feed line of its own. In this instance, indeed the actuation pressure for the axial feed movement, and therewith also the axial feed speed, as well as the knife pressure, can be adjusted sensitively; however the relevant knife holder requires, as has been said, two feed lines. Also, the triggering point in time for the axial feed movement is not satisfactorily fixed, since the reversing valve operates as a function of the variable pressure in the feed line of the drive member for the radial feed movement, which in turn depends upon the relevant restoring resilient force. In addition, with the pressure forces which are effective on both sides also elastic forces are in equilibrium with one another, which, in conjunction with the unavoidable friction forces, have a tendency to cause a slip-stick effect.